Chemisorbed atoms and molecules of reactants as active sites in heterogeneous catalysis

Article abstract of DOI:10.1007/s10975-005-0099-3

In some heterogeneous chemical processes occurring on the solid/gas interface, reactant atoms and molecules chemisorbed during the reaction are not directly involved in chemical transformations but act as catalytic sites accelerating this reaction.

Full text of DOI:10.1007/s10975-005-0099-3

Kinetics and Catalysis, Vol. 46, No. 4, 2005, pp. 464–471. Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 497–505.
Original Russian Text Copyright © 2005 by Kharlamov.
Chemisorbed Atoms and Molecules of Reactants
As Active Sites in Heterogeneous Catalysis
V. F. Kharlamov
Orel State Technical University, Orel, Russia
Received December 8, 2003
Abstract—In some heterogeneous chemical processes occurring on the solid/gas interface, reactant atoms and
molecules chemisorbed during the reaction are not directly involved in chemical transformations but act as cat-
alytic sites accelerating this reaction.
In situ investigation of the mechanisms of heteroge-
Our kinetic studies of atomic adsorption with con-
neous reactions at solid/gas interfaces is carried out by tinuous determination of the rate of the heterogeneous
kinetic spectroscopic methods. In these methods, reac- recombination of adsorbed atoms have demonstrated
tion rate is measured along with the rate of transforma- that relationships (1) are not satisfied [6–9]. The dis-
tion of surface compounds resulting from the chemi- agreement between these relationships and similar data
sorption of reactants during the reaction [1]. The reported by other authors [10] was discussed earlier [7,
chemisorption of gas molecules alters the state of the 11]. According to [6–9], the heterogeneous recombina-
surface, changing the probabilities of chemical reac- tion of hydrogen or oxygen atoms on the surface of var-
ious solids at n ≈ 1014 cm–3 proceeds by the mechanism
tions involving adsorbed species. For example, chemi-
sorbed molecules can participate in energy transfer and
in the stabilization of highly reactive intermediates or
product molecules, thereby raising the efficiency of the
catalyst.
2R + 2Z
RZ + RZ
R2 + 2Z, where R is an atom
arriving from the gas phase, Z is an adsorption site,
and RZ is a physically adsorbed atom. In the case of
hydrogen or oxygen atomic beam in high vacuum (n <
109 cm–3), the intensity of the radical-recombination
luminescence (RRL) of crystal phosphors is I ~ nN [4,
5], suggesting that the luminescence is excited owing to
Here, we report experimental evidence for the accel-
eration of the heterogeneous reactions H + H
H2
and CO + O
CO2 by chemisorbed reactants.
the reaction R + (RZ)
R2 + Z or RZ + (RZ)
Numerous studies have been devoted to the mecha-
nism of the heterogeneous recombination of dissoci-
ated gas species and accompanying phenomena,
including the luminescence of crystal phosphors and
particle emission. The results of these studies have been
interpreted in terms of the Eley–Rideal (ER) or Lang-
muir–Hinshelwood (LH) mechanism [2–5]. The fol-
lowing relationships are observed for the ER and LH
mechanisms of the heterogeneous recombination of
R2 + 2Z [7, 12], where (RZ) is a chemisorbed atom.
Thus, depending on the experimental conditions, the
rate of a heterogeneous reaction can be controlled by
different elementary steps. By studying the reaction in
regimes differing in the rate-determining step, it is pos-
sible to derive a deeper insight into the reaction mech-
anism. No such studies have been devoted to the heter-
ogeneous recombination of radicals. The purpose of
our study was to use and develop the above approach.
Experiments were carried out at various concentrations
of active gas species (6 × 1012 cm–3 ꢀ n ꢀ 1015 cm−3) and
various temperatures (293 K ≤ T ≤ 518 K). Further-
more, we developed a method enabling one to establish
and control, in one run, different reaction regimes by
modulating the flux of reacting molecules.
atoms, R + R
R2 [6, 7]:
J = AnN + BN2;
τ < N*(J*)–1,
(1)
where J is the rate of the reaction (formation of product
molecules), Ä and Ç are coefficients, n is the concentra-
tion of atoms in the gas medium, N is the concentration
of chemisorbed atoms, and τ is the relaxation time for
the occupation of the surface by chemisorbed atoms
after the appearance of atoms in the gas medium. Here-
after, stars designate steady-state values. According to
the author’s knowledge, most of the publications deal-
ing with the heterogeneous recombination of atoms
report no direct validation of at least one of relation-
EXPERIMENTAL
The catalysts were ultrafine copper powder (spheri-
cal particles with an average diameter of 100 nm); fine-
particle ZnS–Cu, ZnS–Tm, and CaO–Mn phosphors
with a specific surface area of ~1 m2/g; and Pt/Al2O3
ships (1), while reaction mechanisms deduced from (0.6% Pt) with a specific surface area of ~10 m2/g.
indirect data are questionable.
Hydrogen and carbon dioxide were spectroscopic-
0023-1584/05/4604-0464 © 2005 MAIK “Nauka /Interperiodica”

CHEMISORBED ATOMS AND MOLECULES OF REACTANTS AS ACTIVE SITES
465
grade. Carbon dioxide was obtained by the thermal
decomposition of CaCO3 in vacuo and was dried using
a silica gel column. Hydrogen (99.995%) was produced
in a GVCh-6 hydrogen generator and was also dried in
a silica gel column. The dissociation of gas molecules
10
8
(H2
2H and CO2
CO + O) was carried out by
9
producing high-frequency electric discharges between
electrodes placed 0.4 to 1.0 m upstream of a flow cata-
lytic reactor. Under our experimental conditions, disso-
ciated carbon dioxide consisted of CO2 and nearly
equal amounts of CO and O [13].
G1
4
G2
Elementary chemical processes on solid surfaces in
reactive media were studied by relaxation measure-
ments [9, 14–16]. These measurements provided infor-
mation as to the participation of free, chemisorbed, and
physically adsorbed species in surface reactions.
Simultaneously, we recorded the time dependences of
the concentration of adsorbed reacting species, N(t), of
the dynamic effect of reaction (DER) [7, 17], F(t) =
[Pr(t) – P]S = GJ(t), and of the intensity of the RRL of
crystal phosphors, I(t) = ηJ(t). Here, t is time, Pr is gas
pressure at the catalyst surface, P is gas pressure on the
walls of the vessel, S is the surface area occupied by the
catalyst, G is a quantity depending on the momentum
distribution of gas species in the plane of catalyst sur-
face, η is RRL efficiency, and J is the rate of formation
of product molecules. Absolute values of J were not
determined, and the J(t) dependence was studied by
indirect measurements. In the case of crystal phos-
phors, reaction rate was evaluated by two independent
methods, specifically, by measuring DER (F) and RRL
intensity (I), which are proportional to J.
2
1
5
6
N
S
3
7
G3
Fig. 1. Schematic of the experimental setup. See text for
the key.
quartz resonator with a sensitivity limit of 4.4 × 10−9 g.
Both surfaces of the piezoelectric element (4) were
covered with a layer (of thickness d ≈ 0.1 mm) of an
ethanolic suspension of the ultradisperse substance to
be examined. The concentration of chemisorbed spe-
cies, N, was judged from the decrease in the resonator
frequency, f, which was measured with a ChZ-54 fre-
quency meter: N ~ ∆f; ∆f = f(t1) – f, where f(t1) is the ini-
tial oscillation frequency of the piezoelectric element.
(An increase ∆x in the thickness of the quartz plate of
the piezoelectric element as a result of adsorption is
known to change the frequency of the quartz resonator
The flux of reactive gas species was modulated by
three methods using different techniques. In the first
method, gas species were allowed to flow for ~102 s.
This time was sufficiently long for an adsorption equi-
librium to be established on the catalyst’s surface. After
that, the flow was turned off [9]. The second method
was used to study the interaction between the solid and
“packets” of reactive gas species moving with the car-
rier gas. The transit time of these packets in the reactor
was ~1 s. This time was too short for the surface to
undergo any change. These periodically introduced
packets were superimposed on a constant flow of reac-
tive species, which was turned on for ~102 s [14]. In the
third method, the surface was probed with a high-den-
sity packet of active species carried by a gas stream. To
diminish the diffusion spread of the packets, the flow
velocity was greatly increased. This enabled us to lower
the time resolution threshold of the method by more
than one order of magnitude (to 10 ms) and to study the
kinetics of the heterogeneous process over time inter-
vals of 0.1 s [15, 16].
by ∆f = f 2kf–1 ∆x, where kf is the frequency factor.) The
temperature coefficient of frequency of the resonator
was 1 Hz/K. The measurement error arising from the
heatup of the piezoelectric sensor by the chemical reac-
tion did not exceed 5%. The adsorption capacity of the
piezoelectric sensor surface (at d = 0) was negligible.
The dynamic effect of reaction (in newtons) was
measured using an automated balance with magnetic
suspension [18]. The sensitivity of this balance, which
had been calibrated against a weight of 1 mg, was 2.5 ×
10–6 g, and the time constant was 10–1 s. The signal-to-
noise ratio was 10 to 103. The measurement error did
not exceed 10%. An ethanolic suspension of the sub-
stance to be examined was applied onto the upper sur-
face of a substrate (5) to form a layer of thickness d ≈
0.1 mm. The substrate was placed near the piezoelectric
element and was attached to the moving part of the bal-
Gas at a pressure of P = 30 Pa was continuously
pumped through discharge tubes (1–3) and a glass vac-
uum chamber (flow reactor) as shown in Fig. 1. The
walls of the chamber could be heated to 500 K. Adsorp- ance with the use of a quartz filament. DER can be rep-
tion (in relative units) was measured using a piezoelectric resented as F = F0 + Fg [7, 17]. Here, F0 is the gas pres-
KINETICS AND CATALYSIS Vol. 46 No. 4 2005

466
∆f × 10–2, Hz
KHARLAMOV
F × 107, N
reactor and, thereby, lost their excess energy [19]. No
charged species entered the reactor—this was ascer-
tained experimentally. Discharge radiation was
absorbed using a Wood horn. The reactor received a
mixture of molecules and radicals in the ground elec-
tronic and vibrational states from the discharge tubes.
The adsorption and recombination of hydrogen atoms
on the walls of the discharge tube and reactor and on the
sample surface did not produce any effect on the con-
centration of H atoms in the gas phase or on the kinetics
of the processes examined [10, 16].
6
4
2
0
15
2'
1
10
5
1'
2
When pulse modulation of the flux of active species
was used, the concentration of H atoms entering the
reactor (in relative units) was determined from the
pulsed intensity of the blue glow line of the hydrogen
atom (λ = 487 nm), as described in [20]. This line was
separated from the glow spectrum of H using a UM-2
monochromator. Hydrogen glow intensity in the
plasma (Ig) and the intensity of RRL of crystal phos-
phors (I) during the heterogeneous reaction were mea-
sured with two FEU-85A photomultipliers. The electric
current of either photomultiplier, which was propor-
tional to Ig or I, was measured every 12 µs using two
analog-to-digital converters. The numerical data thus
acquired were entered into a PC to immediately obtain
I(t) and Ig(t) curves.
0
0
50
100
150
200 t, s
Fig. 2. Time dependence of (1, 2) the frequency of the
piezoelectric resonator coated with Pt/Al O catalyst and
(1', 2') the dynamic effect of reaction after (↑) initiation and
(↓) quenching of a discharge in (1, 1') carbon dioxide and
(2, 2') hydrogen. T = 350 K.
2
3
sure force that acts on the catalyst surface owing to the
change in the momentum distribution of the particles
flying away from the surface in the course of the reac-
tion and Fg is the pressure force due to the reaction-
induced temperature step at the solid/gas interface. To
reduce the cooling effect of the gas molecules on the
substrate and to increase Fg, the substrate (5) was made
from two 0.1-mm-thick mica plates separated by
0.2 mm. This allowed the conditions F0 ꢁ Fg and
F ≈ Fg [7, 17] to be satisfied.
To remove the adsorbed surface impurities (ethanol,
water, and air molecules), the samples were held in a
monohydrogen–dihydrogen mixture at 350 K for 1.5 h.
The sputtering of the catalyst surface by H atoms was
monitored using a piezoelectric quartz resonator. Once
the sputtering rate fell sharply, the treatment of the sam-
ple with monohydrogen was stopped. In the case of
copper, the surface oxides were reduced by hydrogen
atoms and the black powder turned red. As this took
Gas discharges were produced using two identical
high-frequency generators (G1 and G3 in Fig. 1). Either
generator could produce either a continuous or a pulsed
discharge. The duration of each pulse was 1 s, and the
pulse repetition frequency was 0.02 or 0.04 s–1. The
pulsed discharge periodically added a value of ∆n (spe-
cies packets) to the constant concentration of active
species, n. Either discharge was screened and was inde-
pendent of the other. Discharge power could be varied
between 10 and 70 W [14]. The steady-state concentra-
tion of active gas species over the sample surface in the
reactor, measured by the hot probe method, was
6 × 1012–1 × 1014 cm–3, depending on discharge power.
In other experiments, a pulsed discharge with a pulse
duration of 0.1 s was initiated in the gas using a high-
frequency generator (G2) with a variable power
(0.1−1 kW). This discharge was confined using a capil-
lary (6) and a magnet (7). Simultaneously, using an
electronic control unit (8) and a solenoid valve (9), we
injected, over 0.6 s, a portion of molecular gas from an
auxiliary reservoir (10) with P = 104 Pa into the dis-
charge tube. This procedure yielded a packet of active
species with a density of ∆n ≈ 1015 cm–3, whose transit
time was 0.1 s [15, 16]. The vibrationally and electron-
ically excited molecules that had formed in either dis-
charge zone experienced at least 103 collisions with
place, the dynamic effect of the H + H
H2 reaction
increased by one order of magnitude. The hydrogen
was replaced with carbon dioxide within 2 s while
maintaining the continuous gas flow regime in the reac-
tor. After one gas had been replaced with another, the
reactor was flushed with molecules and a high-fre-
quency discharge was then initiated in the gas.
In blank runs, in which there was no catalyst on the
sensor, the effects described below were not observed.
RESULTS AND DISCUSSION
After a constant flow of active gas species was
established, the occupation of the surfaces of all cata-
lysts by chemisorbed hydrogen atoms or CO and O spe-
cies proceeded at a comparatively slow rate, with a
characteristic relaxation time of τ ≈ 10–100 s. By way
of example, we demonstrate, in Fig. 2, the kinetics of
the adsorption of dissociated carbon dioxide and hydro-
gen atoms on the surface of the Pt/Al2O3 catalyst as the
time dependence of resonator frequency (curves 1, 2).
The reactions O + O
é2 and CO + O
CO2
molecules of the gas medium while diffusing to the occur on the solid surface in the dissociated carbon
KINETICS AND CATALYSIS Vol. 46 No. 4 2005

CHEMISORBED ATOMS AND MOLECULES OF REACTANTS AS ACTIVE SITES
467
Fm × 106, F × 106, N
dioxide medium, the latter reaction dominating. Other-
wise, since oxygen atoms leave the surface owing to the
recombination reaction O + O é2 and the CO
N, arb. units
1
8
2
adsorption energy is high [21], CO molecules would
accumulate on the surface during the heterogeneous
reaction and the adsorption of dissociated carbon diox-
ide at T ≈ 300 K would be to a significant extent irre-
versible. According to experimental data, this is not the
case.
6
2
3
4
1
50 s
2
2 s
For the heterogeneous reaction H + H
H2 to
occur by the ER and LH mechanisms, the condition
χ < 1, where χ ≡ τJ*(N*)–1, must be satisfied [6, 7]. To
estimate χ, we will use the relationship J* = 0.25nνγ,
where ν is the mean velocity of hydrogen atoms
impinging on the surface and γ is the coefficient of the
heterogeneous recombination of hydrogen atoms.
Using observed τ data and γ and N* values known from
the literature, we obtained χ ꢂ 1 for all of the catalysts.
For the platinum catalyst at T = 300 K, γ = 0.025 [2] and
N* ≤ 4.6 × 1014 cm–2 [21]; hence, χ = 104. For ZnS, γ =
3 × 10–3 [3] and N* ≤ 1 × 1014 cm–2 [10, 22]; hence, χ =
103. Comparison of experimental data for the reactions
0
0
t
Fig. 3. Time dependence of (1) F , (2) N, and (3) F for the
m
interaction between a periodic sequence of hydrogen atom
packets and copper surface before and after a constant flux
of H atoms is (↑) turned on and (↓) turned off. T = 293 ä,
n = 1 × 10 cm , and n ∆n = 3. The inset shows the shape
of the DER peak.
13
–3
–1
in the 2CO + é2
catalyst is demonstrated in [27, 28].
2CO2 reaction over the Pt/Al2O3
H + H
H2 and CO + O
CO2 (see, e.g., Fig. 2)
suggests that the χ values in these reactions are compa-
rable. This violation of the condition χ < 1 means that
the ER and LH mechanisms are invalid for these reac-
tions.
The shapes of the DER curve F(t) and the RRL
curve I(t) depend on the concentration of active species
in the gas medium and on the nature of these species.
For n ≈ 1014 cm–3, once a continuous discharge is initi-
ated, F and I grow abruptly and then remain almost
invariable (see, e.g., Fig. 2, curves 1' and 2'), indicating
that the reaction rate is independent of the concentra-
tion of chemisorbed reactants (compare curves 1 and 1',
In all systems investigated, after the discharge is
quenched, the reaction rate falls abruptly at least by two
orders of magnitude and gas desorption is observed
(Fig. 2). The desorption curves are linearized on the t–
t(∆f)–1 coordinates, where ∆f is the time-dependent
change in resonator frequency [13], indicating the
recombinational desorption of chemisorbed species
[22]. Therefore, the chemical reactions in the chemi-
sorbed layer (LH mechanism) make only a negligible
contribution to the rate of the heterogeneous reaction.
In the case of the H + H
H2 reaction, a decrease
and curves 2 and 2'). A possible explanation for this in the concentration of H atoms in the gas medium to
result is that the reaction involves species chemisorbed
n ≈ 1013 cm–3 alters the shape of the F(t) and I(t) curves
on hypothetical active sites, the concentration of these
species being below the detection limit of adsorption
measurements. However, the probability that the sur-
faces of all of the catalysts have sparse catalytic centers
obtained after a continuous discharge is initiated: the
initial portions of these curves indicate a nearly linear
growth of F and I. To illustrate the effect of the concen-
tration of active gas species on the shape of the F(t) and
I(t) curves, we will consider data collected using the
that are very active in both H + H
H2 and CO + O
CO2 reactions is negligible. Furthermore, the cross sec- pulse modulation of atomic flux. The interaction
between the periodic sequence of H packets and the cat-
alyst gives rise to DER peaks and luminescence scintil-
lation (when phosphors are used), whose amplitudes
Fm = F(n + ∆n) and Im = I(n + ∆n) are shown by points
tion for the reaction taking place on these centers
through the ER collision mechanism would exceed the
physically plausible values. Therefore, the above
results can be considered to be evidence in favor of the
reaction occurring by the recombination of species that in Figs. 3–5. First, we will consider the recombination
have been bound by the surface and are in a so-called of H atoms on the surface of the copper catalyst. Under
precursor state [23, 24], which can be viewed as a phys- steady-state conditions (when the observed value does
ically adsorbed state. Similar data indicating that the not change with time), both the concentration of chemi-
rate of the 2CO + é2
2CO2 reaction over platinum sorbed atoms (N*(n)) and DER (F*(n)) increase mono-
tonically with an increasing concentration of hydrogen
atoms in the gas medium. The F*(N*) dependence is
nearly linear: F* ≈ kN*, where k is a coefficient. Under
unsteady-state conditions, the amplitude of the period-
ically emerging DER peaks (Fm) increases once a con-
is independent of the dioxygen coverage of the surface
(0.4 ≤ θ ≤ 1) have been explained by the participation of
weakly bound (precursor) CO molecules as a two-
dimensional gas [25] in the reaction (CO being chemi-
sorbed by platinum does pass through a precursor state
[26]). The participation of the two CO adsorption states tinuous discharge is initiated and then remains invari-
KINETICS AND CATALYSIS Vol. 46 No. 4 2005

468
KHARLAMOV
I, arb. units
Fm × 106, N
F × 106, N
Im, arb. units
1
4
4
1
1'
3
3
2
1
0
1'
100 s
2
1
0
100 s
2
2
2
2
0
0
2'
2'
t
t
Fig. 4. Time dependence of (1, 2) F and (1', 2') F for the
Fig. 5. Time dependence of (1, 2) I and (1', 2') I for the
m
m
interaction between a periodic sequence of hydrogen atom
packets and ZnS–Cu phosphor surface before and after a
constant flux of H atoms is (↑) turned on and (↓) turned off.
interaction between a periodic sequence of hydrogen atom
packets and ZnS–Cu phosphor surface before and after a
constant flux of H atoms is (↑) turned on and (↓) turned off.
13
12
–3
13
12
–3
T = 293 ä; n = (1, 1') 7 × 10 and (2, 2') 6 × 10 cm ;
T = 293 ä; n = (1, 1') 3 × 10 and (2, 2') 6 × 10 cm ;
−1
−1
n ∆n = (1, 1') 0.2 and (2, 2') 6.
n
∆n = (1, 1') 2 and (2, 2') 6.
k3
able (Fig. 3, curve 1). It is independent of the concen-
tration of chemisorbed atoms (N), which increases with
time (Fig. 3, curve 2). After the continuous discharge is
quenched, Fm takes its initial value and recombinative
desorption of H atoms is observed. The shape of the
continuous F(t) curve obtained after a continuous dis-
charge is initiated depends on the concentration of
atoms in the gas medium: at n ꢀ 1 × 1013 cm–3, the F(t)
curve has a portion where F increases together with the
concentration of chemisorbed atoms (Fig. 3, curve 3).
At n ꢃ 3 × 1013 cm–3, the F(t) curve has a rectangular
shape. If the concentration of atoms in a packet (∆n) is
fixed and the constant component of atom concentra-
tion (n) increases (causing an increase in N), then pulse-
induced increments in DER, ∆F = F(n + ∆n) – F(n), do
not increase and ∆F is independent of N. If ∆n is varied
at fixed values of n and N, ∆F grows with increasing ∆n.
2RZ
R2 + 2Z,
R2 + 2Z,
k4
2(RZ)
where (RZ) is a chemisorbed atom and RZ is an atom
in the precursor state. The following rate equations cor-
respond to this model:
2
˙
'
N1 = k1(N0 – N) – k1N – 2k3N1,
2
˙
'
N = k2(N0 – N) – k2N – 2k4N ,
where N1 is the concentration of precursor atoms, N0 is
the concentration of adsorption sites, and k1 – k4 are the
rate constants of the reactions (k1 = 0.25nνσ, where σ
is the capture cross section of free atoms passing into
the precursor state). Assuming that the precursor and
chemisorbed atoms make up “rapid” and “slow” sub-
systems and using the adiabatic approximation, we
obtain
Let us see whether the symbatic time variation of N
and F (Fig. 3, curves 2, 3) can be explained in terms of
the heterogeneous reaction H + H
H2 proceeding
–2
2
2
J ≅ k3N21; J ≈ k3k1(N0 – N) (k1) ,
'
by a mechanism involving chemisorbed hydrogen
atoms (ER and LH mechanisms). For this mechanism,
F = GJ = αnN + βN2, where α and β are coefficients. If
this were the case, changing n several times would not
break down the symbasis of the F(t) and N(t) curves.
Experimental data disprove this hypothesis (compare
curves 1–3). Furthermore, in the case of the copper cat-
alyst, the condition necessary for the participation of
chemisorbed H atoms in chemical reactions (χ < 1) is
not satisfied. Reactions involving physically adsorbed
or chemisorbed atoms are unlikely for the same rea-
sons. In view of this, let us consider the following
model of the reaction:
(2)
(3)
2
'
'
8k1k3(N0 – N) ꢁ (k1) ; 2k3N1 ꢁ k1.
J ≈ 0.5k1(N0 – N),
2
'
8k1k3(N0 – N) ꢂ (k1) ; 2k3N1 ꢂ k1.
˙
˙
Under steady-state conditions (when N1 = N = 0), J* =
–1
–1
2
'
' *
0.5k1k2 k2 N* + (1 + k1k2 )k4(N*) – 0.5k1N1 . At rela-
'
'
tively small values of k4 and k1 (when 2k4N* ꢁ k2 and
'
*
k1 ꢁ 2k3 N1 ), we obtain the relationship J* ≈ kN* (k is
a coefficient), which is in agreement with experimental
data. In the case of N ꢁ N0, Eq. (3) describes the
Fm(t) = GJm(t) and F(t) = GJ(t) dependences observed
for high atom concentrations in the gas medium (Fig. 3,
curve 1). Under steady-state conditions, the linear
k1
R + Z
R + Z
RZ,
'
k1
k2
(RZ),
'
k2
KINETICS AND CATALYSIS Vol. 46 No. 4 2005

CHEMISORBED ATOMS AND MOLECULES OF REACTANTS AS ACTIVE SITES
469
dependence of the reaction rate on the concentration of
atoms in the gas is due to the fact that the recombination
reaction 2RZ R2 + 2Z is controlled by the capture
I, arb. units
Im, arb. units
6
6
of H atoms passing into the precursor state RZ: J ≅
0.5k1N0. Expression (2) corresponds to the E(t) curves
obtained at small n values (Fig. 3, curve 3). The con-
stancy of Fm after the initiation of a continuous dis-
charge (curve 1) indicates that the ascending portion of
the F(t) curve (curve 3) is not due to any uncontrollable
evolution of the state of the surface. Therefore, this
increase in F can be caused only by k3 growing with
time because of the rising concentration of chemi-
sorbed atoms. The buildup of chemisorbed atoms is
likely to be due to their participation in the stabilization
of resulting molecules through energy transfer in the
adsorbed layer according to the scheme 2RZ + (RZ)
R2 + 2Z + (RZ). There are three possible energy-trans-
fer channels for the relaxation of the vibrational energy
localized on a forming H–H bond, namely, the vibra-
tion–vibration channel, the vibration–electronic chan-
nel (which leads to an excited state of the (RZ) bond),
and the vibration–translation channel (which is due to
two-dimensional translations of (RZ)). Note that, as the
concentration of hydrogen atoms in the gas medium
increases, satisfaction of the condition k3 ~ N ~ n facil-
itates a crossover from the reaction regime correspond-
ing to Eq. (2) to the regime corresponding to Eq. (3).
In the recombination of hydrogen atoms on the sur-
face of ZnS–Cu phosphor, the parameters measured
after the initiation of a discharge change with time at a
lower rate than in the case of the copper catalyst, yet
they show similar behavior (Figs. 4, 5). Therefore, the
above reaction mechanism and Eqs. (2) and (3) are
applicable to the recombination of H atoms on zinc sul-
fide surface. As judged from the rectangular shape of
the F(t) curves, Eq. (3) is valid for the copper catalyst
and ZnS–Cu phosphor at n ≈ 1014 cm–3; therefore, the
high catalytic activity of copper as compared to zinc
sulfide [2, 3] is primarily due to the higher capture cross
section (σ) of H atoms passing into the precursor state.
The value of σ is determined by energy removal in the
stabilization of a captured hydrogen atom on the sur-
face. The phonon relaxation of vibrationally excited
states of the gas atom–surface bond is known to pro-
ceed at a comparatively low rate. For metal surfaces,
the conversion of vibrational energy into the energy of
electronic excitation [29] is more probable. It is due to
effective vibronic energy transfer that the capture cross
section of hydrogen atoms passing into the precursor
state is larger for copper than for zinc sulfide.
4
4
2
1'
2
2'
1
2
0
0
0
400
800
1200
t, s
Fig. 6. Time dependence of (1', 2') scintillation intensity for
the ZnS–Cu phosphor interacting with a periodic sequence
of CO and O packets and (1, 2) permanent glow intensity for
the same phosphor before and after a constant flux of CO
–1
and O species is (↑) turned on and (↓) turned off. n ∆n = 6;
T = (1, 1') 293 and (2, 2') 390 K.
I(n + ∆n) is the amplitude of the RRL peaks and ∆N =
N(n + ∆n) – N(n) (Fig. 6, curves 1, 1'). Raising the tem-
perature to 390 K under the same conditions decreases
the amount of chemisorbed CO and O several times,
causing a reduction in Im and a marked decrease in Im/I
(curves 2, 2'). These results cannot be explained by the
participation of chemisorbed species in the heteroge-
neous reaction. We will consider a model in which reac-
tion rate is determined by the recombination of CO and
O in the precursor state. The equality ImI–1 ≈ n–1(n + ∆n)
is valid for curves 2 and 2', implying that J = η–1I ≈
0.5k1N0. This is the highest possible reaction rate.
Therefore, the fact that the reaction rate increases with
decreasing temperature if the other conditions are
invariable (see curves 1' and 2') is explained by the
increasing capture cross section for CO and O passing
into the precursor state. This effect is due to surface
coverage increasing as the temperature is decreased,
considering that chemisorbed species participate in the
removal of excess energy from precursor species, sta-
bilizing them on the surface according to the scheme
R + Z + (RZ)
RZ + (RZ). Here, the most plausible
scenario, in terms of energy, is the relaxation of the
vibrational energy localized on the resulting RZ bond
through the excitation of bending vibrations in chemi-
sorbed species or through translations of (RZ) species
(vibration–vibration and vibration–translation energy
transfer, respectively).
In the case of the heterogeneous reaction CO + O
CO2 occurring on the surface of the ZnS–Cu phosphor
at 295 K, RRL kinetics is affected by O and CO adsorp-
tion, which is extensive and partially irreversible
under the conditions examined. When both the contin-
uous and pulsed sources of active species are turned on
(n–1∆n = 6), the following conditions are satisfied:
ImI−1 > 102 and ImI–1 ꢂ n–1∆n + N–2(∆N)2, where Im =
Let us consider the RRL scintillation of crystal
phosphors under the action of single high-density pack-
ets of hydrogen atoms. For the CaO–Mn crystal phos-
phor, the plot of RRL intensity versus time, I(t), has a
nearly rectangular shape (Fig. 7, curve 2). The I(t)
KINETICS AND CATALYSIS Vol. 46 No. 4 2005

470
Ig, I, arb. units
KHARLAMOV
Im, arb. units
2
4
3
2
1
0
1
2
1
2
1
0
1
2
I g2, arb. units
Fig. 8. RRL scintillation amplitude for the CaO–Mn phos-
0
50
100
150
200
t, ms
phor at T = (1) 345 and (2) 518 K versus the square of I (t)
g
averaged over the discharge pulse duration.
Fig. 7. Time dependence of (1) hydrogen atom glow inten-
sity in the discharge tube and (2) luminescence intensity for
the CaO–Mn phosphor at T = 509 K under the action of a
Similar data indicating that the heterogeneous reac-
tion H + H
H2 responsible for RRL proceeds
through the recombination of precursor atoms are
obtained for the interaction between high-density H
packets and ZnS–Tm and ZnS–Cu phosphors. We
found that the RRL efficiency η of the ZnS–Cu phos-
phor decreases with increasing hydrogen atom concen-
tration in the gas medium. This effect is due to the spe-
cific features of the electronic spectrum of this phos-
phor [16].
15
–3
single hydrogen atom packet. ∆n ≈ 10 cm .
curve is shifted relative to the glow intensity curve for
H atoms in the discharge plasma (Ig(t); Fig.7, curve 1)
by the time it takes for an H packet to travel from the
discharge tube to the reactor (17 ms). As the power of
the pulsed discharge is raised at a fixed pulse duration,
the RRL scintillation amplitude Im for the CaO–Mn
phosphor shows an approximately quadratic depen-
dence on the density of the H packet (Fig. 8). The reli-
ability of the least-squares approximation Im ~ n2 is ρ2 =
0.71 at T = 345 K (curve 1) and ρ2 = 0.83 at T = 518 K
(curve 2). Depending on the power of the pulsed dis-
charge, the ratio of Im to the intensity of RRL due to the
constant flux of H atoms from the generator G3 (Fig. 1),
I*, is 2 to 50. Therefore, the highest density of a hydro-
gen atom packet under our experimental conditions is
∆n ≈ 1015 cm–3.
When gas molecules are chemisorbed on a solid sur-
face, they pass through a precursor state [24].As the gas
pressure is raised, the concentration of molecules in the
precursor state increases and the probability of chemi-
cal reactions involving these molecules grows when-
ever it is possible. However, since relaxation methods
for the study of surface reactions at “high” pressures of
P = (102–105) Pa are not well developed, it is still
unclear whether precursor molecules are involved in
heterogeneous reactions. The results of this study draw
attention to heterogeneous reaction models that take
into consideration the participation of precursor mole-
cules in reaction steps. It is expected that such models
will adequately describe some real chemical processes.
Indeed, the approach that takes into account molecule
capture resulting in a precursor state has provided a
solution for the problem of getting over the pressure gap
in hydrogen oxidation over platinum, 2H2 + é2
2H2é. That is, it has resulted in a unified reaction
model valid for both high vacuum and moderate and
high pressures [30].
According to adsorption measurements, the interac-
tion between H packets and the surface of the CaO–Mn
phosphor results in an increase in the concentration of
chemisorbed species, N. (The maximum surface cover-
age is Nmax ≈ 0.25∆nνtgα1 ~ 1014 cm–2, where tg = 0.1 s
is the time of transit of atom packets through the reactor
and α1 ≈ 10–5 [22] is the sticking coefficient of H atoms
passing into the precursor state.) If chemisorbed hydro-
gen atoms were involved in the heterogeneous recom-
bination responsible for RRL (through the ER and LH
mechanisms), it would be expected that I ≈ Bt + ët2,
where I = ηJ = anN + bN2 (N ~ t; B, C, a, and b are coef-
ficients). The fact that this condition is not satisfied
(Fig. 7) means that the heterogeneous reaction
CONCLUSION
At 293–500 K and active gas species concentrations
of n = 1013–1015 cm–3, the heterogeneous reactions
H + H
H2 proceeds by the recombination of pre-
cursor hydrogen atoms. Since the condition Im ~ n2 is
met at 518 K (Fig. 8), the desorption rate of precursor
atoms at this temperature is high as compared to the
recombination rate of these atoms (see Eq. (2)).
H + H
H2 and CO + O
CO2 on solid (Cu, Pt,
ZnS, and CaO) surfaces proceed through the recombi-
nation of physically adsorbed (precursor) species. The
cross sections of heterogeneous reactions occurring by
KINETICS AND CATALYSIS Vol. 46 No. 4 2005

CHEMISORBED ATOMS AND MOLECULES OF REACTANTS AS ACTIVE SITES
471
3. Styrov, V.V., Kinet. Katal., 1968, vol. 9, no. 1, p. 124.
4. Grankin, V.P., Styrov, V.V., and Tyurin, Yu.I., Kinet.
Katal., 1983, vol. 24, no. 1, p. 141.
the Eley–Rideal collisional mechanism are negligible
in this case, and the reactions in the chemisorbed layer
(Langmuir–Hinshelwood adsorption mechanism) do
not make any considerable contribution to the reaction
rate. Depending on the concentration of active gas spe-
cies, temperature, and the nature of the catalyst, the rate
of these reactions is limited either by the capture of inci-
dent gas species with the formation of precursor species
or by the recombination of precursor species. The partic-
ipation of precursor species in the heterogeneous reac-
tion is most obvious in the case of “high” concentrations
of these species in the gas medium (n ꢃ 1014 cm–3).
These results were obtained by independent methods.
5. Grankin, V.P. and Tyurin, Yu.I., Kinet. Katal., 1996,
vol. 37, no. 4, p. 608.
6. Kharlamov, V.F., Khim. Fiz., 1994, vol. 13, no. 6, p. 83.
7. Kharlamov, V.F., Rekombinatsiya atomov na poverkh-
nosti tverdykh tel i soputstvuyushchie effekty (Recombi-
nation of Atoms on Solid Surface and Accompanying
Effects), Tomsk: Tomsk. Gos. Univ., 1994.
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Pis’ma Zh. Tekh. Fiz., 1998, vol. 24, no. 5, p. 23.
9. Kharlamov, V.F. and Anufriev, K.M., Pis’ma Zh. Tekh.
Fiz., 1999, vol. 25, no. 15, p. 27.
10. Styrov, V.V., Yagnova, L.I., and Izmailov, Sh.L., Kinet.
Katal., 1975, vol. 16, no. 3, p. 705.
For “low” concentrations of hydrogen atoms in the
gas medium (n ꢀ 1013 cm–3), the concentration of
chemisorbed hydrogen atoms and the rate of the heter-
11. Kharlamov, V.F., Khim. Fiz., 1991, vol. 10, no. 8,
ogeneous reaction H + H
H2 vary with time in the
p. 1084.
same way. This symbasis is not explained by the partic-
ipation of chemisorbed H atoms in the reaction. It is due
to the fact that chemisorbed monohydrogen alters the
state of the surface, increasing its catalytic activity. This
increase probably arises from the stabilization of result-
ing hydrogen molecules through energy transfer taking
place during the reaction according to the scheme
12. Kharlamov, V.F., Zh. Fiz. Khim., 1997, vol. 71, no. 4,
p. 678.
13. Kharlamov, V.F. and Rogozhina, T.S., Zh. Fiz. Khim.,
2003, vol. 77, no. 4, p. 632.
14. Kharlamov, V.F., Rogozhina, T.S., Barmin, A.V., et al.,
Pis’ma Zh. Tekh. Fiz., 2002, vol. 28, no. 13, p. 67.
15. Makushev, I.A., Barmin, A.V., Kharlamov, V.F., et al.,
Prib. Tekh. Eksp., 2003, no. 1, p. 134.
16. Kharlamov, V.F., Makushev, I.A., Barmin, A.V., et al.,
Pis’ma Zh. Tekh. Fiz., 2003, vol. 29, no. 7, p. 87.
17. Kharlamov, V.F., Poverkhnost, 1993, no. 11, p. 122.
HZ + HZ + (HZ)
H2 + 2Z + (HZ). This is accom-
panied by a relaxation of the vibrational energy of the
forming H–H bond in the H2 molecule through the
excitation of the (HZ) bond. The similar enhancing
effect of CO and O chemisorption on the catalytic activ-
ity of zinc sulfide surface in the heterogeneous reaction
18. Anufriev, K.M., Kharlamov, V.F., and Razumov, A.V.,
Prib. Tekh. Eksp., 2000, no. 1, p. 152.
CO + O
CO2 is due to the increase in capture cross
19. Callear, A.B. and Lambert, J.D., in Comprehensive
Chemical Kinetics, vol. 4: The Formation and Decay of
Excited Species, Bamford, C. and Tipper, C., Eds.,
Amsterdam: Elsevier, 1969.
section for the particles reacting on the surface and
passing into the precursor state according to the scheme
R + Z + (RZ)
RZ + (RZ). The most likely cause for
the stabilization of surface-captured species is the con-
version of vibrational energy of the forming bond RZ
into the energy of bending vibrations of the (RZ) bond.
20. Zaitsev, V.V., Opt. Spektrosk., 1992, vol. 72, no. 4,
p. 859.
21. Boreskov, G.K., Kataliz: voprosy teorii i praktiki (The-
ory and Practice of Catalysis), Novosibirsk: Nauka,
1987.
Metals (Cu and Pt) show a higher catalytic activity
in the H + H
H2 reaction than nonmetals (ZnS and
CaO) [2, 3] (see Eq. (3)). This is primarily due to the
difference in capture cross section for hydrogen atoms
passing into the precursor state. According to theory
[29], the larger values of σ for metals may be due to the
active participation of conduction electrons in energy
removal stabilizing precursor H atoms on the surface.
22. Izmailov, Sh.L. and Kharlamov, V.F., Kinet. Katal.,
1982, vol. 23, no. 5, p. 1179.
23. Kisliuk, P.J., J. Phys. Chem. Soc., 1957, vol. 3, p. 95;
1958, vol. 5, p. 78.
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1992, no. 9, p. 5.
25. Matsushima, T., Almy, D.B., and White, J.M., Surf. Sci.,
1977, vol. 67, no. 1, pp. 89, 122.
ACKNOWLEDGMENTS
26. Weinberg, W.H., Comrie, C.M., and Lambert, R.M.,
The author is grateful to T.S. Rogozhina,A.V. Barmin,
and I.A. Makushev for their assistance in this study.
J. Catal., 1976, vol. 41, no. 3, p. 489.
27. Kiperman, S.L., Gaidai, N.A., Nekrasov, N.V., et al.,
Chem. Eng. Sci., 1999, no. 54, p. 4305.
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KINETICS AND CATALYSIS Vol. 46 No. 4 2005